Stressed structural element



April 25, 1%? H. J. KAHN STRESSED STRUCTURAL ELEMENT 3 Sheets-Sheet 1Filed Aug. 19, 1963 INVENTOR. HARRY J. KAHN wwm his ATTORNEYS P 167 H.J. KAHN STRESSED STRUCTURAL ELEMENT Filed Aug. 19, 1965 3 Sheets-Sheet 2HARRY J. KAHN his ATTORNEYS April 25, 1967 J K 3,315,425

STRESSED STRUCTURAL ELEMENT Filed Aug. 19 1963 3 Sheets-Sheet 5 INVENTORHARRY J. KAHN his ATTORNEYS United States Patent 3,315,425 STRESSEDSTRUCTURAL ELEMENT Harry J. Kahn, 188-49A 71st Crescent, Fresh Meadows,N.Y. 11365 Filed Aug. 19, 1963, Ser. No. 302,913 13 Claims. (Cl. 52223)of 72,000 pounds, are being subjected to live loads as great as 106,000pounds. Efforts to control this dangerous situation through load limitnotices, traffic laws, truck weighing stations and police supervisionhave failed. As an alternative to police control, strengthening thesestructures in accordance with conventional engineering prac tices wouldbe prohibitively expensive.

Further engineering problems are presented by the current aestheticallyand economically inspired trend toward applying materials other thansteel to structural tasks. These new materials, such as aluminum, failto exhibit their optimum structural potential when treated according toconventional structural steel design practices. Thus for a specificdesign load, the necessarily rigid application of traditional beamequations strictly determine the size, shape and weight of thestructural member, leaving to the designer only a small margin foringenuity, improvement, or individual expression.

Continuous beams, in theory stronger and much stiffer than simple beams,are subject to marked changes in bending stress, reaction and shear bysmall unequal supporting pier subsidences. The stress changes resultingfrom unequal pier subsidence produce a highly overstressed beam whichmust be avoided.

The present invention provides a method and apparatus for reinforcingexisting structures and overcoming the disadvantages imposed on newstructural materials by traditional engineering design concepts and isparticularly adapted to all materials deemed structural.

In accordance with the present invention a stressing device having meansfor storing measured torsional forces is attached to two or morestructural members, and the stored torsional forces are transferred fromthe storage means to the structure, whereby the size, shape and Weightof the structural members are substantially reduced as compared tounstressed structural members designed to support an equivalent load.The torsional force is applied to prestress or load the structuralmembers in a direction opposite from the expected direction of designedstructural load application. Consequently, as a design load is imposedon the structure, the prestressing force, as well as the inherentunstressed load bearing capacity of the structure, must be reachedbefore the members are fully loaded.

More particularly, the aforementioned means for storing the measuredtorsional forces, according to the invention may comprise a pair ofcoaxial torsion members such as tubes, rod, shafts, bars or the likejoined on one end by a transverse connecting plate from which anattachment plate is spaced axially and fixed to the one member while asecond attachment plate, also axially spaced from the connecting plate,is attached to the other element. A stressing link applies a measuredtorsional force by relatively twisting the elements about their commonlongitudinal axis. Couple arm beams, in engagement with the upper andlower flanges of abutting structural members, are each pivoted torespective attachment plates and fulcrumed over one of the torsionmembers. Releasing the stressing link permits the torsion members tounwind, transferring the torsional forces, as a couple, through thecouple arm beams to the abutting structural members to prestress orspring the members. may also comprise a vertical torsional member orseries of concentric vertical members as later described which exhibitsgreater efiiciencies depending upon space limitations and application.

The invention accordingly provides an economical method and apparatusfor increasing the load bearing capacity of present structures byprestressing existing structural components, or enabling selectivereduction of beam size in new construction for a given design loadcondition, according to the magnitude of the prestressing force chosenby the designer. The application of this prestressing technique to newand to existing structures is subject to conventional methods ofstructural analysis and does not require the use of new mathematicaldesign concepts, as will be explained. By reducing the overall size ofthe structural members for a given design load, the method and apparatusprovided by the present invention, substantially reduce manufacturing,transportation and installation costs and thereby increase the economicattractiveness of new structural materials. By springing the members ina continuous beam, according to the invention, overstressing conditionscreated by foundation settling are naturally reduced as are stressesfrom moving loads.

For a more complete understanding of the present invent, reference maybe had to the accompanying drawings in which:

FIGURE 1 is a front elevational view of one embodiment of the stressingdevice according to the invention as installed in a conventional highwaybridge;

FIG. 2 is an enlarged front elevation-a1 view in partial section of thestressing device shown in FIGURE 1;

FIG. 3 is a side elevation in full section of the stressing device shownin FIG. 2 as seen along the line 33 of FIG. 2;

FIG. 4 is a top plan view in full section of the stressing device shownin FIG. 2, as seen along the line 4-4- of FIG. 2;

FIG. 5 is a front elevation of another apparatus for practicing theinvention;

FIG. 6 is a top plan view in partial section as seen along the line 66of FIG. 5;

FIG. 7 is a plan view in partial section, as seen along the line 7-7 ofFIG 5; and 7 FIG. 8 is a sectional view of a modified form of stressingdevice including a coiled or helical spring.

Referring to FIG. 1, a typical torsional force storage means orstressing device 10 embodying the invention is interposed between theabutting ends of two conventional structural members or beams 11 and 12.Beam 12 is supported on a pier or foundation 13 by a fixed bearing 15,while the beam 11 rests on a transversely slidable expansion bearing 16,also resting on pier 13.

The stressing device 10, according to one embodiment of the invention,shown in enlarged FIG. 2, has an inner torsion member 17 such as a bar,rod, shaft, or as illustrated, a tubular member telescoped within anouter tubular member 18. The inner and the outer tubular members 17 and18 are joined at one end by a transverse annular connecting plate 19best shown 'in FIG. -3 and welded or similarly secured to thecorresponding ends of the tubular members 17 and 18.

As shown in FIGS. 3 and 4, unconnected ends of the inner and outertubular members 17 and 18 spaced axially from the annular connectingplate 19 are disposed The storing device nearest the abutting ends ofthe structural members 11 and 12 and are each respectively fixed to aninner attachment means or plate 20 and an outer attachment means orplate 21.

The couple arm beam 23 transfers measured loadings (FIG. 2) from theprestressing device to an upper flange 24 and a lower flange 25 of thestructural member 12. A pair of flange fittings 26 and 27 secured byscrews 28, or some similar means, to the inwardly opposing faces of theflanges 24 and 25 provide abutting surfaces for the vertical extremitiesof the couple arm beam 23 through which a couple, established by thetorsional force stored within the stressing device 10, can betransferred to spring or prestress the structural member 12. The outerattachment plate 21 and couple arm beam 34 similarly springs thestructural member 11 through upper and lower flanges 29 and 30, flangefittings 31 and 32. Bolts 22 and 33 (FIGS. 2 and 3) are a means ofreacting the dead weight of assemblies 10 and 44.

A stressing link 38, comprising two oppositely threaded members 39 and40 transversely disposed below the inner tubular member 17 and spacedbetween the inner and outer attachment plates 20 and 21 protrudesthrough transverse apertures formed in the lower portions of the couplearm beams 23 and 34 respectively. Flanged head portions 41 and 42,interposed between the couple arm beams 23 and 34 and the lower flangefittings 27 and 32, terminate the extreme ends of the threaded members39 and 40 to lock the couple arm beams 23 and 34 in joint, inwardlypivoting motion. The inner, mutually opposed ends of the members 39 and40 are threadedly engaged in a turnbuckle 43.

Accordingly, in order to twist the inner tubular member 17 and the outertubular member 18 relative to each other and store the torsional forcewithin the device, the turnbuckle 43 is tightened to draw the threadedmembers 39 and 40 and the lower portions of the couple arm beams 23 and34 together (FIG. 2) to the predetermined prestressing value. It will beobserved that since the couple arm beams, pivoted to the inner and outerattachment plates 20 and 21, and in tangential contact with the innertubular member 17 are fulc-rumed against the inner tubular member 17,this arrangement further enhances the spring like action of thestressing device 10.

The torsional stress characteristics of the storage device 10 are suchthat, within the safe shearing stress of the tube material, the twist orangular deflection of the tubular members 17 and 18 is generallyproportion-a1 to the applied load. Consequently, a linear relationshipexists between the work input to the turnbuckle 43 on the stressing link38 and the torsional force established within the tubular members 17 and18.

A similarly constructed torsional force storage device 44 can beinstalled (FIGS. 3 and 4) on the opposite side of the webs 45 and 46 tobalance the stress imposed on the beam flanges by the stressing device10.

In practice, the foundation or pier 13, supporting the two abuttingbeams 11 and 12 is suitably dimensioned at the cap to provide suflicientarea needed to mount the wider spaced bearings.

The relationship between the torsionalforce established within thetubular members 17 and 18 and the linear separ-ation of the lower endsof the couple arm beams 23 and 34 established by the stressing ink 38can be determined by suitable calibration tests.

A specific torsional force is applied to the stressing device 10 bytightening the turnbuckle 43, drawing the opposing inner ends of thethreaded members 39 and 40 together a predetermined distanceproportional to the force to be stored within the prestressing device10. Shims 47 are interposed between the flange fittings and the couplearm beams to match the transverse dimensions of the couple arm beams 23and 34 in a torsionally stressed device to the transverse separation ofthe abutting. surfaces of the flange fittings 26, 27, 31 and 32.

The device 10 is inserted between the abutting ends of the beams 11 and12 in engagement with the flange fittings and shimmed into properadjustment while the companion device 44 is similarly applied (FIGS. 3and 4) to the structural members 11 and 12 on the other side of the webs45 and 46.

The turnbuckles are loosened, freeing the beams to press against theflange fittings under the torsional force released within the tubularmembers. The mechanical force couples so released from the stressingdevice 10 are transferred to the structural members 11 and 12, to springor prestress the beams. The force couples applied to the structuralmembers 11 and 12 prestress the beams against imposed loadings and makeit possible to reduce the beam size required to support such loadingsrelative to an unstressed beam designed to support equivalent loadingsor to support greater loads than a similar unstressed beam.

The torsional'force storage device 10 may become a: part of the finalassembly in structures using simple beams. This permanent incorporationof the stressing device 10 in simple beam structures offers the furtheradvantage of availability to react to some degree additional loadsimposed on the element thereby reducing both stress and, deflection.

The torsional force storage device 10 can also be used to apply aspecific prestressing force in continuous beam structures. In some casesof continuous beams, after application of the prestressing force to theindividual stringers, the stringers are interconnected to produce acontinuousbeam, for example as in a highway bridge, by pouring theconcrete deck and interconnecting the individual stringers. When thecontinuous beam is assembled, the torsional force storage device 10 isremoved by tightening the prestressing link 38 and withdrawing thedevice from abutting interposition between the flange fittings.

As hereinbefore mentioned, the formulae governing the physicalcharacteristics of beams prestressed in accordance with the presentinvention are developed from conventional structural engineeringequations. Values may be chosen by the designer, such that the mosteconomical sizes of device and beam members are obtained. Thus thesigificant dimensions of a torsional force storage device similar to thedevice described and illustrated, can be determined from the followinganalysis:

Where GI L=GI IR Where G=angle of rotation in radians L=length oftorsion device in inches G=modulus of rigidity in p.s.i.

R=rate of device in inch pounds per radian The length and polar momentof inertia are adjusted for the characteristics of the individual tubes,bars, or shapes. The material and physical properties of the device, ifmade for example, from steel or aluminum, can be chosen in accordancewith design requirements and derived through the methods advanced inPlastic Torsional Buckling Strength of Cylinders Including the Effectsof lrnperfections, by L. H. N. Lee and C. S. Ades.

When the device is of the concentric horizontal configuration, due topractical design considerations, according to the embodiment of theinvention illustrated in FIGS. 1 to 4, inclusive, the tubular members 17and 18 have different lengths and different polar moments of inertia.

Rearranging the basic equations for angular rotation, 9, the inner andouter tube lengths 17 and 18 are:

L17=GI 17917/T 1s= p1s 1a T Assuming B to be the additional length ofthe inner tube 17, the tube lengths may be equated:

L17=L18+B GI 9 /T=GI 9 /T+B The twists of the two members must equate tothe known or total twist of the assembly:

Revising this equation in terms of the inner member length 17=( 17 1sT-l- 17+ 1s) A singular or multiple vertical device would abide by thesame basic torsional equations with slight modification to the twistequations of this page.

The physical characteristics of structural members prestressed by adevice made and applied in accordance with present invention can becomputed by conventional analytical procedures.

The general differential equation of the elastic curve of a beam may bewritten:

where M is the bending moment equation in terms of the span length (x).Applying, by way of example, this fundamental equation for beamstiffness to a simple beam of negligible weight subjected topre-twisting two stressing devices 10, each fixed to the ends of thebeam, at x=0, y= and x=l, y=0, respectively, the equation of the bendingmoment to the left of any point of examination, P, remains constant atthe value of the moment AM imposed by the stressing device 10 at x=0,y=0. Both support reactions, R and R are zero. Thus:

The differential equation for the specific beam is:

r5 11 T I EI :62 Ali 0 Integrating this differential equation twice withrespect to x yields:

Using the boundary conditions at x=0, y=0 and the boundary conditions atx: 1, :0 the deflection equation may be evaluated: v

The maximum deflection occurs at the centerline of the beam, or x=6/2,and is:

Ely: -AM06/4(6/26) y=AM 6 /8EI The general equation for twist is thevalue of dy/dx, or: 9EI=EI(dy/dx)=AM x+AM 6/2=-AM (x-6/2) The twist atthe stressing device, where x=0, is:

G=AM 6/2EI A further embodiment of the invention, shown in FIGS.

5, 6 and 7, can be used to prestress such structural elements asstructural members 50 and 51 for use with buildings, bridges, towers,traveling cranes and the like. In terms of craneways, the members maycarry steel capped tracks 52 and 53 on upper flanges 54 and 55. Verticalwebs 56 and 57 on the members 50 and 51 are .provided with abuttingrecessed slots 58 and 59 accommodating torsional force storage means 60.The torsional force storage means 60 has a torsion member of tubular,bar or rod-like shape 61 made or metal or the like, vertically disposedwithin the recessed slots 58 and 59. Attachment means 62 and 63, havingupper horizontal couple arm beams 64 and 65 (FIG. 6) and lower couplearm beams 66 and 67 (FIG. 7) are secured by welding or some similarmeans to the vertical extremities of the torsion member 61. The lowercouple arm beams 66 and 67 extend transversely across adjacentrespective lower flanges 68 and 69 (FIG. 7) to engage abutting lowerfiange fittings 70 and 71, fastened by bolts 72, or similarly secured tothe inner faces of the flanges 68 and 69. The upper couple arm beams 64and 65 are similarly engaged (FIG. 5) with flange fittings 73 and 74 tothe upper flanges 54 and 55. The engagement between the couple arrnbeams and the flange fittings is such that the upper couple arm beams 64and 65 place the upper flange fittings 74 and 75 in tension while thelower couple arm beams 66 and 67 place the lower flange fittings 70 and71 in compression.

In operation the torsional force storage means 60 is twisted about thevertical axis through a pre-selected angle to store a specific amount ofwork within the torsion member 61. The upper and lower couple arm beams64, 65 and66, 67 are placed in abutting relationship with the respectiveopposing flange fittings 70, 71, 73 and 74 and the torsional forcesstored within member 61 are released. The upper and lower couple armbeams 64, 65, 66 and 67 rotate in opposite directions to tension theupper flange fittings and compress the lower flange fittings therebyestablishing two force couples in the horizontal plane. Each couple actsto prestress or spring one of the respective members 50 and 51.

FIG. 8 discloses a prestressing device similar to the device disclosedin FIGS. 1 to 4 in which the tubular member 17 is replaced by a coiledor helical spring having one end fixed to an attachment plate 81 and theother end connected to another attachment plate 82 fixed to the end of atubular member 83. The plate 81 and the adjacent end of the member 83are connected to couple arm beams 84 and 85, respectively. This deviceoperates to prestress the beams in the same way as the device of FIGS. 1to 4 inasmuch as the spring 80 or the spring and the tubular member 83act as torsional stress storing members.

As is apparent from the foregoing, the present invention provides amethod and apparatus for prestressing structural members tosubstantially reduce the overall size and weight of the membersnecessary to support a specific load.

While a representative embodiment of the present invention has beenshown and described for purposes of illustration, various changes andmodifications can be made therein as pointed out above without departingfrom the principles of this invention. Therefore, all such changes andmodifications are included within the intended scope of the invention asdefined by the following claims.

I claim:

1. A device for prestressing structural members, comprising a resilienttorsional force storage means for cooperative engagement with at leasttwo of the structural members, a releasable stressing link, at least twoportions on the torsional force storage means adapted to be operativelyconnected to the stressing link for the selective establishment of aresilient torsionally stressing couple, and attachment means operativelyconnected to the storage means for transferring the couple from theresilient torsional force storage means to the structural members whenthe stressing link is released.

2. A device according to claim 1 wherein said torsional force storagemeans includes a coiled spring.

3. A device according to claim 1 wherein said torsional force storagemeans includes a helical spring. 1

4. A device according to claim 1 wherein said torsional force storagemeans includes a generally cylindrical shell having an inner and anouter tubular member resiliently joined together on one end thereof.

5. A device for prestressing structural members, cm-

prising a generally cylindrical shell having an inner and an outertubular member resiliently joined together on one end thereof by atransverse connecting plate for cooperation with at least two of thestructural members, a releasable stressing link operatively connected tothe inner tubular member and to the outer tubular member for theselective establishment of a torsionally stressing couple within thecylindrical shell, and attachment means adapted to transfer the couplefrom the cylindrical shell to the structural members when the stressinglink is released.

6. A device according to claim 5, wherein the attachment means includesan outer transverse attachment plate spaced longitudinally from thetransverse connecting plate and in fixed cooperation with the outertubular member, an inner transverse attachment plate spacedlongitudinally from the transverse connecting plate and in fixedcooperation with the inner tubular member, a pair of transversely spacedcouple arm beams, each of said couple arm beams operatively fastened toone of the transverse attachment plates, and fittings secured to thestructural member flanges adapted to cooperatively receive said couplearm beams for transferring the couple from the cylindrical shell to thestructural members when the stressing link is released.

7. A device for prestressing structural members, comprising a pair ofconcentrically arranged tubes having an inner tube, protruding beyondone end of the outer tube, a trasverse connecting plate operativelyjoining the inner and outer concentric tubes at one end, an outer tubeattachment plate fixed to the other end of said outer tube and spacedfrom the transverse connecting plate,

an inner tube attachment plate spaced from the outer tube attachmentplate and fixed to the protruding end of the inner tube, a pair oftransversely spaced couple arm beams cooperatively interposed betweensaid inner, tube attachment plate and said outer tube attachment plate,fastening means pivotally attaching one of said couple arm beams to theinner tube attachment plate and the other of said couple arm beams tothe outer tube attachment plate in tangential engagement with the innertube, a stressing link operatively connecting the pair of couple armbeams for selectively establishing a torsional stress-' ing couplewithin the inner and outer tubes, and fitting means on the structuralmembers adapted to abut the couple arm beams to transfer the couple fromsaid inner' and outer tubes to the structural members when saidstressing link is released.

8. A stressing device according to claim 1 including a plurality oftorsional force storage means cooperatively engaging the structuralmembers.

9. A method for stressing abutting structural members, comprisingangularly twisting a, tubular member, locking the angular twist into thetubular member, attaching one end of said tubular member to one of theabutting structural members, attaching the other end of said tubularmember to another of the abutting structural members, releasing theangular twist in the tubular member, and transferring said angular twistforces to prestress the abutting structural members.

10. A method according to claim 9, including fastening the prestressedstructural members together, and removing said tubular member from thetwisted structural members.

11. A device for prestressing abutting structural members, comprising agenerally cylindrical member interposed between at least two of theabutting structural members, at least two pairs of couple arm beamsfixed to the transverse extremities of said tubular member, at least twopairs of flange fittings cooperatively engaging said respective couplearm beams with the structural members, one of said pairs of couple armbeams compressing said respective pair of flange fittings and another ofsaid pairs of couple arm beams tensioningsaid respective pair of flangefittings to pivot vertically the abutting structural members.

12. A device for prestressing abutting structural members, comprising agenerally vertical cylinder on multiple coaxial cylinders for storingtorsional forces therewithin interposed between the abutting structuralmembers, an upper couple arm beam extending transversely across thestructural members and operatively fixed to the upper extremity of saidvertical cylinder, a lower couple arm beam extending transversely acrossthe structural members and operatively'fixed to the lower extremity ofsaid vertical cylinder, a pair of upper flange fittings cooperativelytransferring tension forces from said upper couple arm beam to thestructural members, and a pair of lower flange fittings cooperativelytransferring compression forces from said lower couple arm beam to thestructural members toprestress the structural members.

13. A stressing device according to claim 12 including a plurality oftorsional force storage means cooperatively engaging the structuralmembers.

- References Cited by the Examiner Cheskin 52-223 FRANK L. ABBOTT,Primary Examiner. R. S. VERMUT, Assistant Examiner.

12. A DEVICE FOR PRESTRESSING ABUTTING STRUCTURAL MEMBERS, COMPRISING A GENERALLY VERTICAL CYLINDER ON MULTIPLE COAXIAL CYLINDERS FOR STORING TORSIONAL FORCES THEREWITHIN INTERPOSED BETWEEN THE ABUTTING STRUCTURAL MEMBERS, AN UPPER COUPLE ARM BEAM EXTENDING TRANSVERSELY ACROSS THE STRUCTURAL MEMBERS AND OPERATIVELY FIXED TO THE UPPER EXTREMITY OF SAID VERTICAL CYLINDER, A LOWER COUPLE ARM BEAM EXTENDING TRANSVERSELY ACROSS THE STRUCTURAL MEMBERS AND OPERATIVELY FIXED TO THE LOWER EXTREMITY OF SAID VERTICAL CYLINDER, A PAIR OF UPPER FLANGE FITTINGS COOPERATIVELY TRANSFERRING TENSION FORCES FROM SAID UPPER COUPLE ARM BEAM TO THE STRUCTURAL MEMBERS, AND A PAIR OF LOWER FLANGE FITTINGS COOPERATIVELY TRANSFERRING COMPRES- 